A method for the preparation of the (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid by a four-step synthesis is known from the prior art (see RU 2452490 C1, publ. Oct. 6, 2012):

Amyl ester of carbamic acid is synthesized. Phenyl isocyanate (11.5 g-0.096 mol) is mixed with pentanol (amyl alcohol) (8.5 g-0.096 mol), the reaction mass is heated, and after 1 hour the mixture solidifies in the form of colorless crystals. The yield of amyl ester of carbamic acid is about 100%.

Carbopentoxysulfanilic acid chloride is synthesized. To chlorosulfonic acid (17.5 g to 0.150 mol) heated to 30° C., amyl ether of phenylcarbamic acid (2.07 g-0.010 moles) is slowly added while stirring, maintaining the temperature of the reaction mass at a level no higher than 35° C. Then the mixture is slowly heated to 50° C. and kept at a temperature of 50-55° C. for 2 hours. The resulting sulphomass is poured onto ice while stirring, keeping the temperature below 20° C. The precipitate is filtered off, washed with ice water up to the pH of filtrate 7, dried in air and then in a desiccator. The yield of carbopentoxy sulfanilic acid chloride is about 100%.

(2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid is synthesized. To a mixture of 2,6-dichloroaniline (2.2 g to 0.0136 mol) and pyridine (3.23 g-0.0406 mol) carbopentoxysulfanilic acid chloride (6.2 g-0.0203 mol) is added in portions at the temperature of 85° C.; the reaction mass is stirred at 80° C. for 45 minutes. Then 20 ml of hot water are added to the mass, the mixture is acidified with hydrochloric acid to pH 3-4 and cooled to the room temperature. The resulting precipitate is filtered off, washed with water until the odor of pyridine disappears, and dried. After recrystallization, the yield of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid is about 40%.

The final product, the sodium salt of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid is synthesized. 0.232 g of sodium hydroxide (NaOH) are dissolved in 5 ml of ethyl alcohol (C2H5OH), and 2.5 g of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid are dissolved in 30 ml of ethyl alcohol (C2H5OH). Then, the two solutions are mixed and stirred for 20 minutes, after which the ethyl alcohol is distilled off under vacuum. The remaining precipitate is dried. The product yield was 2.1 g (80%). As a result of the process, the sodium salt of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid is obtained.
The disadvantage of this method, accepted as the prototype of the present invention, is the low overall yield of the final product, sodium salt of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid, which does not exceed 32% as yields of the intermediate products and the final product at individual stages are 100, 100, 40 and 80%, respectively. Thus, the total yield of sodium salt of (2,6-dichlorophenyl)amide of carbopentoxysulfanilic acid does not exceed 32%.